(1) Field of the Invention
The present invention relates to a noise shaper, and more particularly to a noise shaper which prevents the occurrence of noise in a low frequency band.
(2) Description of the Related Art
FIG. 1 shows a conventional noise shaper of a three-value output type of the kind to which the present invention relates. The noise shaper 10 shown includes a first integrator 20 formed by a delay circuit 21, a second integrator 40 formed by a delay circuit 41, a quantizing circuit 60 which receives an output of the second integrator 40 and outputs a three-value signal (i.e., 0, +1, or -1), and a feedback circuit 70 for feeding back an output of the quantizing circuit 60 to the first and second integrators 20 and 40. The feedback circuit 70 includes a delay circuit 72 and an amplifier 71.
The operation of the above conventional noise shaper is explained with reference to FIG. 1. The first integrator 20 receives and integrates a difference between an output signal Y of the quantizing circuit 60 and an input signal X supplied to an input terminal 11 of the noise shaper 10. The second integrator 40 receives and integrates a difference between an output signal of the first integrator 20 and a signal resulting from the output signal of the quantizing circuit 60 being amplified by 2 by the amplifier 71. The output signal of the second integrator 40 is forwarded to the quantizing circuit 60. Here, the quantizing circuit 60 outputs a signal of "+1" when the input signal applied thereto is larger than +1/2, a signal of "0" when the input signal is between -1/2 and +1/2, and a signal of "1" when the input signal is lower than -1/2.
In the above operations, the relationship between the input signal X and the output signal Y of the noise shaper 10, when the quantizing noise produced in the quantizing circuit 60 is Q, may be expressed by the following equation (1). EQU Y(z)=X(z)+(1-Z.sup.-1).sup.2 .multidot.Q(z) (1)
Thus, the output spectrum developed at the output terminal 12 of the noise shaper 10 shown in FIG. 1 is a spectrum in which a signal produced by the second-order differentiation of a quantizing noise of the noise shaper is superimposed to the input signal. Since the quantizing noise is shaped and superimposed in a high frequency band, the sum of noise components within a signal in-band is substantially reduced.
However, in the conventional noise shaper described above, there is a problem in that, when no signal is inputted to the noise shaper, the spectrum contained in the output signal allows a low frequency component to appear more as the initial value of the first integrator 20 becomes smaller.
More specifically, in the above conventional noise shaper, when no signal is inputted thereto, if the initial data at the delay circuits 21 and 41 in the first and second integrators 20 and 40 is zero, it can be appreciated that the output signal will be zero. On the other hand, if the data at the delay circuits 21, 41 in the integrators 20, 40 at the initial state is not zero but the initial value of the first integrator 20 is, for example, 0.5, then the output Y of the noise shaper 10 repeats +1 and -1 and never becomes zero. If the initial value of the first integrator 20 is smaller than 0.5 and is, for example, 0.1, then the output Y repeats +1, -1, eight times continuously of 0, +1, -1 and eight times continuously of 0 , . . . It can be appreciated from the above that, where the initial value at the first integrator 20 is not zero, the output Y of the noise shaper 10 never becomes zero, and the smaller the initial value at the first integrator 20, the more frequent is the appearance of zero to follow +1 and -1. The problem resulting therefrom is that low frequency components contained in the spectrum of the output is allowed to be increased.